This project will involve efforts to determine the genetic basis and physiological significance of controlled synthesis of the aminoacyl-tRNA synthetases of both Escherichia coli. and Salmonella typhimurium and elucidate the mechanisms by which these tRNA synthetases elicit formation of the initial regulatory signals for controlled expressions of the cognate amino acid biosynthetic operons and those for growth-rate mediated control of stable RNA synthesis. The principal methods of study will include genetically based and/or physiologically imposed altered control responses in cells cultivated under specific growth conditions, along with structural and functional analyses of the regulatory regions for control of these tRNA synthetases and biochemical analyses of regulatory mutants.